The Sentry's abilities derive from an experimental serum that creates a phase-shift in his molecules, causing each atom to step an instant ahead of the current time line. The serum induces a photosynthetic reaction, resulting in a hyper state of consciousness. Though most of his powers and their limits are still unknown, examples he has demonstrated so far are super-strength, super-speed, invulnerability, and flight.

The Sentry can also project energy fields, control light, and has vast psychic and mental forces mainly used for holding his physical powers together, though it is not yet known whether the Sentry can use them the way Professor X and other psychics use theirs; the only psychic abilities he had displayed so far is implanting his memories inside Paul Jenkins' mind and calming the fury of the Hulk.

It may be theorized that the Sentry also has the ability to produce hard-light constructs similar to those of Dazzler's when it was revealed that the Void is a just an expression of his repressed persona, and thus his creation. With the people dubbing The Sentry as the world's most powerful superhero, and with the serum causing a photosynthetic reaction to his body, completely altering his state of consciousness, it is nonetheless conceivable that Sentry's powers are limitless, and may even rival those of the Silver Surfer's and Phoenix's. In effect, the Sentry's powers are seemingly limitless.

Empowered by the enigmatic Professor's secret formula, high school student Robert Reynolds became a superhuman. After trouncing the school bully who had tormented him, Reynolds sewed together a costume and made his debut as the heroic Sentry. One of very few super heroes active during the years just prior to the Fantastic Four's emergence, Sentry gained new importance when the new wave of heroes rose to prominence. Almost instantly deducing Spider-Man's secret identity, Sentry became a role model for the young hero, as well as an ally to the X-Men, an equal to Reed Richards and a friend to the outcast Hulk. The Sentry even battled Dr. Doom alongside the Fantastic Four and defeated his greatest enemy, the General, with the X-Men's aid.

During this period, the Sentry married the love of his life, Lindy, and took the young Scout as his sidekick; but the arrival of the Void, a shadowy monster that exploited its enemy's greatest fears, ended the Sentry's charmed life. The Void nearly killed Scout, drove Hulk into a rampage, and murdered over one million people in Manhattan. Discovering that the Void had been the dark aspect of his own powers, the Sentry teamed with Reed Richards and Dr. Strange to create a system which made Earth's entire population, themselves included, forget all about the Sentry. With the Sentry inactive and forgotten, the Void vanished.

Years later, Robert's memories slowly returned along with the Void, who rampaged across Europe, murdering many members of the Super-Heroes of Europe (SHE). The Sentry contacted his former colleagues, but the only one who recalled their shared history was the Hulk. As the Void returned to Manhattan, the pieces began to fall in place and the Sentry's former friends assembled to defend him and the city. Remembering the dual nature of Robert's powers, Sentry and Reed Richards reactivated the Watchtower, dispelling the Void and making the world forget the Sentry once again; however, Robert's memories of the Sentry soon returned, and he demanded to be placed on the Raft, S.H.I.E.L.D.' high-security prison for super-villains, claiming that he had killed his wife. During a mass breakout at the Raft, Sentry helped several other heroes subdue many of the escaping villains - notably Carnage, whom Sentry ripped in half. Inspired by this adventure, the other heroes soon formed the new Avengers, but Sentry vanished before they could invite him to join them. The Sentry stayed in hiding, but remained a person of great interest to S.H.I.E.L.D. and the Avengers, who were both fascinated by his powers and worried that they may lead to another Scarlet Witch situation.

Eventually, S.H.I.E.L.D. and the Avengers confronted the Sentry with several disturbing facts: The wife he claimed to have killed was alive, and the only reference in the world that could be found of the Sentry was in comic books, stories invented and written down by the imagination of a man. Unable to face these shocking events, the Sentry fled. Resurfacing as Robert Reynolds at his home, the Sentry was again confronted by the Avengers and S.H.I.E.L.D., along with the Inhumans and the X-Men. He cried, saying that he had warned them of the Void's coming and that it was now too late. The group was suddenly attacked by the Void. Robert, along with the help of Emma Frost, discovered that the reason no one remembered him was not because of his own attempts to erase the Void, but because of the intervention and manipulation of his mind. Mastermind, hired by the General, had used the great mental powers the Sentry needed to contain his abilities to force everyone (himself included) to forget of the Sentry's existence. Should the Sentry ever remember his life, then he would be struck by a fear of the devil attacking the world, hence the Void. With this new found knowledge, the Void disappeared, and Emma Frost resurrected the Sentry. The Avengers offered him membership, partly to keep an eye on him and partly because they would need his power, which he accepted.

The Sentry believed that the Void was trapped in the basement of the Watchtower. When he routinely visited him to make sure he was secure, the Void claimed he knew something the Sentry didn't. Robert believed his wife was cheating on him and that she didn't love him. As his psychiatrist, Doctor Cornelius, tried to probe Robert with questions about his origin and about the Void, Robert grew hysterical and attacked an amusement park. There, it was revealed that-- as opposed to what was commonly believed, that Robert Reynolds was the Sentry and the Void was a separate being-- that it was the Sentry who was the separate entity, and Robert who was the Void. The Void fled, and the Sentry explained to Doctor Cornelius that Robert had been bullied as a child.

Hoping to get high, Robert stole the Professor's secret formula and drank it. Instantly, Robert became the Void and the Sentry was created.
The Sentry confronted Doctor Strange, who told him not to approach the facility where he had stolen the serum. The Sentry did, where he suddenly seemed to wake up in a straight jacket, in a mental institute. He was told that his real name was John Williams, and that he had killed Lindy Reynolds. It was explained that he was crazy, that he had killed Lindy because his 'alarm clock' told him to. Robert believed this, until he realised that there was a ring mark on his finger where he had had his wedding ring. Breaking free from the illusion, the Sentry found that the Professor and Doctor Strange had been trying to keep the secret the Void knew from the Sentry through controlling him. Seeking out the Void, the Sentry discovered the secret: The secret serum that had granted the Sentry his powers would have worked on anyone. The Void argued that the world had enough trouble with one Sentry and one Void saving and killing people; it didn't need six and half billion more. Understanding everything, the Sentry hurled the Void into the sun. The Void warned Sentry that he would return, and when he did, he would kill everyone. The Sentry returned home to the Avengers.